Sam and Dean go to the University to see Professor Morrison in the Anthropology department. They ask the meaning of the symbol carved into all four victims' chests, and he tells them that it will take him time to research it. They convince him to hurry by promising to get his housekeeper a green card, and Dean expresses his displeasure to Sam as they leave. Sam tells him that they don't have any other options: Bobby is dead and they have to settle.
Later Professor Morrison calls them in to explain that the symbol carved on the victims' chests is a variation of a Greek symbol found on the temple of the Goddess Harmonia. According to myth, the offspring of Harmonia and Ares were the Amazons, a tribe of female warriors that had no need for men except for procreation. After being impregnated, they would amputate parts of their lovers and then kill them. Sam's own research uncovers that the Amazons eventually made a bargain with Harmonia that turned them into monsters. Not only were they stronger, but also they could reproduce quickly: within 36 hours of impregnation they could birth children that grew up just as fast until they reached adolescence. Because their mating cycle is every two years, which lines up with when the murders have taken place now and in the past, and because of the strange things Dean has seen at Lydia's house, they have to consider that Lydia used Dean to become pregnant and that Emma is his daughter.
Sam and Dean later start digging through Bobby's files looking for information about the Amazons and Harmonia. Dean puts some papers down on the bed and turns to talk to Sam. When he turns back, the papers have moved, uncovering a piece of paper with Greek script on it. Dean is convinced that Bobby's spirit is in the room with them and that he uncovered the paper on purpose, but Sam thinks it was the wind from the open window that moved the papers. The nearby power lines also make it impossible to trust the EMF reader, which is redlining in the room. Still, Dean convinces Sam to take the paper to Professor Morrison so that he can decipher it.
Morrison translates the Greek script for Sam. According to the paper, it was not the Amazonian women who killed their lovers, but the children born of their mating. They would kill their own fathers as a rite of passage. Once Sam learns the truth, he rushes out of the professor's office and tries to call Dean, but is stopped by Detective Charlene, who throws him across the hall and then unsheathes her own blade, possibly with the intention of killing Professor Morrison. Before she can kill Sam, Sam turns and shoots her in the chest, killing her. Unfortunately, Sam's phone was smashed when he was thrown and he can't call Dean to warn him about Emma. He races to the motel, just in time to kill Emma.
Sam sent the professor, a recording of Arthur Swensen chanting "K'uhul ajaw, Cacao, shi-jiiy -- K'uhul ajaw, Cacao, shi-jiiy.." The professor determined that the language is ancient Mayan and translates as "The Divine god Cacao is born." Morrison also asks Sam whether he (i.e. Sam's alias as an FBI agent) is looking into getting him a job as a technical adviser.
While Sam and Kevin are trying to translate the proto-elamite that Kevin has translated the angel tablets into, Dean asks if they have consulted Professor Morrison. Sam informs Dean that the professor has moved to Papua New Guinea to study the Trobriand Islanders.